Tunable ferroelectric thin film devices for microwave applications

In this thesis, the main work includes improvement of dielectric properties of BST thin films, microwave characterization of ferroelectric thin films, design and fabrication of tunable m

TUNABLE FERROELECTRIC THIN FILM DEVICES FOR MICROWAVE APPLICATIONS

SHENG SU

NATIONAL UNIVERSITY OF SINGAPORE

2011

TUNABLE FERROELECTRIC THIN FILM DEVICES

FOR MICROWAVE APPLICATIONS

SHENG SU (M Sc., Wuhan University)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE

2011

Acknowledgments

Many people have been helping and supporting me in different ways throughout this work I would like to express my deepest gratitude to my supervisor, Professor Ong Chong Kim Working with you has made my Ph.D study a memorable experience In addition to the knowledge and skills you taught me, your serious attitude for scientific research and work ethic has given me a great example of how to work as a scientist I am grateful that you gave me a lot of freedom to pursue what I was interested in Also, I would like

to thank Professor Sow Chorng Haur and Professor Ariando for serving on my thesis committee

Special thanks must be given to Mr Cheng Wei Ning and Dr Wang Peng for their initial introduction in the field of microwave measurement, Mr Chen Xin and Dr Zhang Xiao Yu for their assistance and contribution in this project Many thanks go to my past and present colleagues of the Center of Superconducting and Magnetic Materials (CSMM), Dr Ma Yun Gui, Dr Xu Feng, Dr Zhao Li, Ms Phua Li Xian, Dr Nguyen Nguyen Phuoc, Mr Le Thanh Hung, Ms Song Qing, Ms Lim Siew Leng Their friendship made my graduate study in Singapore more meaningful and enjoyable

Thanks go to all the other people not mentioned here, but to whom I am grateful for their kind assistance in one way or another

Last but not least, I want to express my gratitude and love to my parents,

my wife Wang Fen and my lovely son Sheng Hao Yu, for their encouragement and patience with me and their unending support and love To them this thesis

is dedicated

Table of Contents

Page

Acknowledgments i

Table of Contents ii

Summary v

List of Publications vii

List of Tables viii

List of Figures ix

Chapter 1 Introduction 1

1.1 Motivations for ferroelectric tunable microwave devices 1

1.2 An overview of tunable microwave devices 3

1.2.1 Brief review of non-ferroelectric technologies 4

1.2.2 Ferroelectric technology 6

1.3 Ferroelectric materials and their microwave applications 7

1.3.1 Theory of dielectric response of ferroelectric materials 9

1.3.2 Tunable ferroelectric thin film microwave devices 13

1.4 Objectives of this study 15

References 17

2 Fabrication and Microwave Characterization of Ferroelectric Thin Films 21

2.1 Fabrication of ferroelectric thin films 21

2.1.1 Pulsed laser deposition process 23

2.1.2 Target preparation and thin film deposition parameter

optimization 25

2.2 Microwave measurement techniques for ferroelectric thin films 26

2.2.1 Lumped capacitance measurement method 27

2.2.2 Coplanar waveguide transmission line method 31

2.2.3 Coplanar resonator method 34

2.3 Experimental measurement 36

2.3.1 Preparation of top electrode layer 36

2.3.2 Measurement setup 39

References 39

3 Ferroelectric Thin Film Varactors 42

3.1 Introduction 42

3.2 Parallel plate varactors 44

3.2.1 Characterization of microwave dielectric properties of BST parallel plate varactors 44

3.2.2 Effects of bottom electrodes on microwave dielectric properties of BST parallel plate varactors 54

3.3 Comparison of microwave properties of BST varactors with parallel plate and interdigital electrodes 59

3.4 Hybrid varactors 67

3.4.1 Design with coplanar and parallel plate structures 67

3.4.2 Experiments and results 69

3.4.3 Conclusion 72

References 72

4 Coplanar Waveguide Ferroelectric Thin Film Microwave Phase Shifters 76

4.1 Introduction 76

4.1.1 Microwave phase shifters 76

4.1.2 Coplanar waveguide transmission lines 78

4.2 Theory of distributed transmission line phase shifters 80

4.3 Design and implementation of coplanar waveguide ferroelectric microwave phase shifters 82

4.4 Experimental results and discussion 87

4.5 Conclusion 88

References 89

5 Coupled Microstrip Line Ferroelectric Thin Film Microwave Phase Shifters 92

5.1 Properties of coupled microstrip line 92

5.2 Design and simulation of coupled microstrip line phase shifter

circuit 94

5.3 Fabrication process and measurement results 100

5.4 Conclusion 103

References 104

6 Composite Right/Left-Handed Transmission Line Microwave Phase Shifter Using Ferroelectric Varactors 105

6.1 Introduction 105

6.2 Model of composite right/left-handed transmission line phase

shifters 107

6.2.1 Left-handed transmission lines 107

6.2.2 Composite right/left-handed transmission lines 108

6.2.3 Phase shift of varactor-tuned CRLH TLs 109

6.3 Realization of CRLH TL line phase shifter using ferroelectric varactors 111

6.3.1 Design and fabrication 111

6.3.2 Measurement results and discussion 113

6.4 Conclusions 117

References 118

7 Dual-Tunable Trilayered Structure of Ferroelectrics and Multiferroics for Microwave Device Applications 120

7.1 Introduction 120

7.2 Experimental procedure and samples 122

7.3 Results and discussion 123

7.4 Conclusion 128

References 129

8 Conclusions and Future Work 131

8.1 Conclusions 131

8.2 Future work 134

Summary

Recent researches have been focused on the development of microwave tunable devices based on ferroelectric thin films Barium strontium titanate (BaxSr1−xTiO3 or BST) thin films are currently considered to be the most suitable candidate for tunable microwave applications In this thesis, the main work includes improvement of dielectric properties of BST thin films, microwave characterization of ferroelectric thin films, design and fabrication

of tunable microwave devices based on BST thin films

There are little reliable data on the microwave dielectric properties of parallel plate ferroelectric varactors, due to the difficulty of completely removing the parasitic inductance and resistance generated by the electrodes

By the consistency of the electromagnetic simulation results and measured results with simple analytical model, we developed an accurate evaluation technique for microwave dielectric properties which is indispensable for optimizing ferroelectric materials The parasitic effects can be effectively corrected by introducing correction resistances and equivalent circuits

We have studied the microwave dielectric properties of BST thin films deposited by pulsed laser deposition (PLD) Firstly, effects of bottom electrodes including La0.7Sr0.3MnO3 (LSMO), Pt and Au, on microwave dielectric properties of BST parallel plate varactors were investigated Secondly, a systematic comparison of the microwave properties of BST thin film varactors with parallel plate and interdigital electrodes was carried out Finally, a multiferroic trilayered structure composed of a BiFeO3 (BFO) layer and two Ba0.25Sr0.75TiO3 (BST) layers grown on platinized silicon substrate

was studied The significant tuning response for the dielectric constant with the electric field and the magnetic field respectively was obtained for the trilayered structure

In our study on ferroelectric varactors, a new hybrid varactor structure proposed by our group was modified and fabricated In this structure, an ultra-thin film with low conductivity is used as dc bias electrode and at the same time the electrode does not contribute in the electric field distribution of microwave signal The fabricated BST hybrid varactor with a modified structure showed a low capacitance and improved tunability compared with the conventional coplanar varactor

In the development of phase shifter device, we proposed three kinds of phase shifters integrated on high resistance Si-substrates using ferroelectric thin film varactors Firstly, a distributed coplanar waveguide (CPW) transmission line phase shifter using parallel-plate BST thin film varactors was presented This phase shifter structure provided a simple method and high microwave phase shift properties Then, an analog microwave phase shifter, which consists of coupled microstrip line loaded with parallel plate BST varactors and two planar Marchand baluns, was demonstrated The phase shifter devices based on coupled microstrip line structure are less sensitive to interfacial effects and require simple processing Lastly, a composite right/left-handed transmission line (CRLH TL) phase shifter with parallel plate BST thin film varactors was presented The CRLH TL phase shifter using BST varactors provided a differential phase shift with flat frequency dependence characteristic in the operating frequency range

List of Publications

1 S Sheng, and C K Ong, Multifunctional dual-tunable multiferroic

Ba0.25Sr0.75TiO3-BiFeO3-Ba0.25Sr0.75TiO3 trilayered structure for tunable

microwave applications, J Phys D: Appl Phys 44, 165406 (2011)

2 S Sheng, and C K Ong, Distributed transmission line phase shifter using

parallel-plate ferroelectric thin film varactors, Microelectronic Engineering,

87, 1932 (2010)

3 S Sheng, X Y Zhang, P Wang, and C K Ong, Effect of bottom electrodes on dielectric properties of high frequency Ba0.5Sr0.5TiO3 parallel

plate varactor, Thin Solid Films, 518, 2864 (2010)

4 S Sheng, P Wang, X Chen, X Y Zhang, and C K Ong, Two paralleled

Ba0.25Sr0.75TiO3 ferroelectric varactors series connected coplanar waveguide

microwave phase shifter, J Appl Phys 105, 114509 (2009)

5 X Y Zhang, Q Song, F Xu, S Sheng, P Wang and C K Ong, Dielectric dispersion of BaxSr1-xTiO3 thin film with parallel-plate and coplanar

interdigital electrodes, J Phys D: Appl Phys 42, 065411 (2009)

6 S Sheng, P Wang, and C K Ong, Compact Tunable periodically LC

Loaded phase shifter using left-handed transmission line, Microwave and

Optical Technology Letters 51(9), 2127 (2009)

7 X Y Zhang, P Wang, S Sheng, Y G Ma, F Xu and C K Ong, A novel structure for dc bias on varactors in composite right/left-handed transmission lines phase shifter using Ba0.25Sr0.75TiO3 thin film, J Phys D:

Appl Phys 42, 175103 (2009)

8 S Sheng, P Wang, X Y Zhang, and C K Ong, Characterization of

microwave dielectric properties of ferroelectric parallel plate varactor, J

Phys D: Appl Phys 42, 015501 (2009)

9 X Y Zhang, P Wang, S Sheng, F Xu, and C K Ong, Ferroelectric

BaxSr1 −xTiO3 thin-film varactors with parallel plate and interdigital

electrodes for microwave applications, J Appl Phys 104, 124110 (2008)

10 S Sheng, P Wang, and C K Ong, A new hybrid ferroelectric varactor

with promising microwave properties for tunable microwave applications,

IEEE Electron Device Letters, Submitted

11 S Sheng, and C K Ong, Coupled microstrip line microwave phase shifter

using ferroelectric thin film varactors, Progress In Electromagnetics

Research-PIER, Submitted

List of Tables

Page

Table 3.1 The parameters and dielectric properties of the parallel

plate and interdigital varactors

60

Table 4.1 The primary parameters used to simulate phase shifter 83

List of Figures

Page Figure 1.1 ABO3 perovskite lattice structures in paraelectric (a)

and ferroelectric states (b) Above T c, their crystal lattice has a cubic structure in (a) and is paraelectric

Below T c, the centers of the positive and negative charges shift and the crystal is characterized by spontaneous polarization

9

Figure 1.2 Polarization dependences on the applied electric field

for (a) a ferroelectric state and (b) a paraelectric state

In (a), P s is the saturation polarization, P r is the

remnant polarization, E c is the coercive field

Figure 2.4 Schematic of a coplanar waveguide (CPW) on

ferroelectric thin film

Figure 3.1 The cross section of the varactor (a) and the

photograph of the measurement setup using a GSG coplanar waveguide probe (b)

45

Figure 3.2 X-ray diffraction pattern (a) and cross sectional SEM

image (b) of the BST film on Pt/Si substrate

46

Figure 3.3 Measured S 11 reflection data curve of varactor, plotted

in polar representation (a) and phase of the S 11 as a function of frequency for measured data and HFSS simulation results, inset shows the distribution of electric field in varactor using HFSS (b)

48

Figure 3.4 A simplified equivalent circuit of the parallel plate

varactor

49

Figure 3.5 The simulated and measured real and imaginary parts

of the S 11 parameters of the varactor

50

Figure 3.6 Frequency dependencies of capacitance (a) and loss

tangents (b) of varactor before and after correction

The two test structures: a1=20 μm, a2=30 μm, b=60

μm

51

Figure 3.7 Frequency dependences of capacitance of varactor at

V=0 and 12V Inset shows the voltage dependence of

capacitance of varactor at 1GHz

52

Figure 3.8 X-ray diffraction patterns of the BST films on

different bottom electrode

55

Figure 3.9 Cross section SEM images of BST/LSMO (a) BST/Pt

(b) BST/Au (c) varactors

56

Figure 3.10 Dielectric constant and loss tangent of the BST thin

films grown on LSMO, Au, and Pt bottom electrodes

as a function of the measured frequency

57

Figure 3.11 Permittivity of BST film as a function of temperature

at f=10 kHz The inset shows the inverse of

permittivity with the temperature

60

Figure 3.12 Capacitance as a function of frequency for BST film

under 0-60 V dc bias The inset in (a) displays the schematic of interdigital capacitance and the inset in (b) shows the loss tangent with a variation in frequency at different dc voltage

61

Figure 3.13 S 11 and phase shift as a function of frequency for

HFSS simulation results and sample BST-0.25 The inset shows the distribution of electric field in interdigital varactor using HFSS

63

Figure 3.14 dc bias field dependent capacitance of BST film in the

interdigital structure at the frequency of 1 GHz

64

Figure 3.15 (a) Capacitance with a variation of frequency in

parallel plate structure measurement under 0 and 12 V

(b) Loss tangent as a function of frequency at different

dc voltage

65

Figure 3.16 The conceptual diagram of columnar BST grain, grain

boundary and electric field lines in coplanar and parallel-plate varactors

66

Figure 3.17 Layer structure of the hybrid varactor and orientation

of electrical field lines in the varactor

68

Figure 3.18 XRD patterns of the thin films (a) ZnO, (b) BST/ZnO

on (001) LAO substrates

69

Figure 3.19 Photograph of the hybrid varactor The marked

interdigital electrode was used for one-port measurement and the outer electrode was designed for applying dc bias

70

Figure 3.20 Tunability of the hybrid varactor at different bias

voltages Inset shows the capacitance and the quality factor of the varactor at zero bias

71

Figure 4.1 Equivalent circuit of distributed ferroelectric phase

shifter

80

Figure 4.2 (a) Layout of the distributed transmission line phase

shifter of coplanar waveguide (b) The side view of a parallel plate varactor (c) The photograph of top view

of a varactor

82

Figure 4.3 The matching impedance of phase shifter circuit as a

function of capacitance of shunt varactor

84

Figure 4.4 The simulation results of the phase shifter under

different full port impedance

84

Figure 4.5 Frequency dependent insertion loss and return loss of

the distributed ferroelectric phase shifter based on 6 unit cells at different bias voltages

86

Figure 4.6 Differential phase shift of the phase shifter as a

function of frequency at different bias voltages

86

Figure 5.1 A configuration of conventional coupled microstrip

lines

92

Figure 5.2 Field distributions resulting from (a) even-mode and

(b) odd-mode excitation of the coupled microstrip lines

93

Figure 5.3 A schematic structure of a segment of the coupled

microstrip line loaded with three varactors

96

Figure 5.4 The simulated response of the coupled microstrip line

loaded with eight capacitors of capacitance of 0.12 pF

96

Figure 5.5 The topology of the planar balun using microstrip line

to coupled microstrip lines

97

Figure 5.6 Layout of top conductor layer of the planar Marchand

balun

98

Figure 5.7 Simulated S-parameters for the planar Marchand

balun Inset shows magnitude of the electric field in the balun

99

Figure 5.8 Layout of the coupled microstrip line phase shifter,

with all dimensions in millimeters

100

Figure 5.9 Measured frequency dependent insertion loss and

return loss of the phase shifter based on 8 unit cells at zero bias voltages

101

Figure 5.10 Measured differential phase shift of the phase shifter

as a function of frequency at different bias voltages

Figure 6.3 (a) Equivalent circuit of the CRLH TL phase shifter

(b) Layout of the phase shifter (c) and (d) Magnified top-view and cross section of the varactors CR and CL, and RF ground

112

Figure 6.4 Capacitance and loss tangent of two series connected

varactors at f=1 GHz as a function of dc bias up to 10

V

114

Figure 6.5 Comparison between measured and HFSS simulated

forward transmission and reflection magnitude

114

Figure 6.6 Measured magnitude and phase responses of the phase

shifter under different bias voltages

115

Figure 6.7 Differential phase shift under different dc biases over

each varactor

116

Figure 7.1 XRD patterns of the films (a) BST, (b) BFO/BST, and

(c) BST/BFO/BST on (111) Pt/TiO2 /SiO2/Si substrates

123

Figure 7.2 SEM picture of cross-section of the trilayered

BST/BFO/BST thin films

124

Figure 7.3 (a) The relative dielectric constant (ε r) versus external

electric fields for the trilayered BST/BFO/BST

structure at 0.1 MHz (b) The P-E hysteresis loops for

the trilayered BST/BFO/BST structure at a series of external fields at room temperature and 250 Hz, where

the inset for the P-E loop of the single layer BFO thin

film

125

Figure 7.4 Microwave frequency dependences of the capacitance

and loss tangent for the BST/BFO/BST trilayered structure The insets show the cross-section of the trilayered structure and the photograph of the measurement setup using a GSG coplanar waveguide probe, respectively

126

Figure 7.5 The capacitance of the multiferroic BST/BFO/BST

trilayered structure versus frequency under different external magnetic field The inset shows the magnetic field dependences of the capacitance at different frequency and loss tangent at 10 kHz

128

Chapter 1

INTRODUCTION

1.1 Motivations for Ferroelectric Tunable Microwave Devices

Modern radio frequency (RF) and microwave engineering is an exciting and dynamic field, due in large part to the explosive growth of commercial wireless markets and the symbiosis between recent advances in modern electronic device technology The widespread use of RF and microwave integrated circuit (IC) technology along with device miniaturization trend have led to the development of RF and microwave circuit components whose dimensions are much smaller than their wavelength The miniaturization, reliability, ease of assembly and compactness of IC fabrication technology are the factors that paved the way for embedding these components directly into the substrates In recent years, there is a rapidly growing demand for electrically tunable RF and microwave devices in advanced radar and mobile communication systems The high dielectric nonlinearity (i.e the strong dependence of dielectric constant on electric field) of ferroelectric materials with perovskite structure has made them promising candidates for these applications

The application of ferroelectric materials in tunable microwave devices was first introduced in the 1960’s [1-4] However, real applications of ferroelectric materials were limited by device electronics and material technology at that time Currently there is a huge research interest in utilizing

ferroelectric thin films for tunable microwave devices since they have high tunability, low loss, fast switching speeds and good power handling capability

at GHz frequencies There are several reviews on different aspects of tunable ferroelectric devices, including both material science and device designs [5-11] Ferroelectric materials are widely used in microwave tunable components such as variable capacitors (i.e varactors), tunable resonators, frequency-agile filters, phase shifters, variable power dividers and tunable oscillators There are also ferroelectric devices based nonlinear components such as harmonic generator, parametric amplifier, pulse shaper and mixer In all the ferroelectric tunable devices, ferroelectric materials always show up in the form of varactors directly or equivalently, so ferroelectric based varactors are crucial components in RF and microwave devices

The BaxSr1−xTiO3 (0 < x < 1) (BST) thin films are currently considered to

be important materials for tunable microwave devices because BST thin films have high dielectric constants, high tunability, low dielectric loss tangent and low leakage current; most importantly, it can be integrated on a traditional Si substrate to replace the current silicon oxide and nitride dielectrics, which would have considerable commercial impact Both theoretical and experimental works have shown that the dielectric constant of BaxSr1−xTiO3 is highly dependent on the temperature, the Ba/Sr ratio and internal or external stresses By changing one or more factors, the dielectric properties can be tuned broadly, especially in the layered or graded composites It is well known that in the vicinity of the paraelectric-to-ferroelectric phase transition temperature, the thermodynamic properties of BST show large anomalies accompanied with large increases in dielectric constant and tunability

The subsequent sections will provide an overview of tunable microwave devices Also, the basic theory of dielectric response of ferroelectric materials and tunable ferroelectric thin film microwave device applications will be presented

1.2 An Overview of Tunable Microwave Devices

The components and circuits in a RF and microwave system can be divided into two groups: passive and active Microwave tunable passive devices mainly include filters, phase shifters, delay lines and network matching circuits in connection with such applications as antenna arrays, communications and radar transceivers In the early days of microwaves, tuning was done manually or mechanically Today, many circuit options are available to realize such tunable devices whose performances are closely related to the choice of a technology These options essentially include mechanical tuning, semiconductors, RF micro-electro-mechanical systems (MEMS), ferrites and ferroelectric materials Electric and magnetic fields (voltage, current), optical interactions or mechanical manipulations are used to achieve tunability in components based on them The ranges and the speed of tunability, control power consumption, losses of microwave signal, power handling capability, potentials of integration, cost, and other parameters of the devices depend on the materials used, the controlling mechanisms (magnetic, optical, electrical, mechanical), and the design The next two sections will briefly review those technologies and put emphasis on ferroelectric material

technology, which has performances comparable or better than competing commercial technologies

1.2.1 Brief Review of Non-Ferroelectric Technologies

The earliest forms of tunable circuits were all mechanical, for example, the rotary vane adjustable waveguide phase shifter firstly proposed by Fox in

1947 [12] Early mechanically tunable devices make use of coaxial lines or hollow metal waveguides and trimming screws/motors/stepper motors Mechanical circuits are inexpensive, easy to fabricate and have very low loss and possess good power handling capability However their disadvantages include their large size, low tuning speed, low cost effectiveness, and sensitive

to vibrations

Semiconductor technology is a very popular alternative for making tunable integrated microwave devices Classically, the tuning can be made with switching or continuous mode using PIN or varactor diodes, respectively [13-14] The tunable devices based on semiconductor are very small (in μms), very fast (<1 μs for pin diode and <1 ns for field effect transistor (FET)), and have large tunability In addition, they can be easily integrated with other circuits for example in monolithic microwave integrated circuits (MMICs) However, their linear decrease of quality factor (Q factor) over frequency confines their usability to be only in the lower end of microwave range Efforts to compensate the high ohmic loss of varactor diodes by using FETs as negative resistors have resulted in almost lossless filters at the expense of higher power consumption and lower bandwidth [15] Despite these problems,

semiconductors are widely used in tuning applications as they offer low cost and compact advantages and allow faster tuning speeds However, for large arrays, such as phased arrays with up to 10 000 and more radiating elements, the power consumption and heat sink are the main problems hindering applications of semiconductor devices

In early 1990s, MEMS were started to use for tunable circuits, where tunability is obtained by the physical movement of a component which changes the capacitance of the device [16] Excluding MEMS antennas, there are to date two generic types of MEMS circuits: switch and varactor [17] MEMS varactors are very competitive [18- 19] They have advantage of very high Q factor, higher self resonance, higher power handling capability, and low control power solutions Moreover, electro-statically actuated MEMS have a near zero power consumption and a more linear capacitance variation with applied voltage However, due to the mechanical structures, their response is slower than ferroelectric and GaAs varactors MEMS also require very high operating voltage by far and they are sensitive to environmental conditions such as moisture, temperature and vibrations

Ferrite has been used to fabricate microwave tunable devices [20-21], mostly include phase shifters and filters These devices take advantage of a property of ferromagnetic materials to change its permeability with an applied

DC magnetic field therefore allowing control of the phase constant of the waveguiding medium Similar to MEMS varactors, ferrite devices can be tuned continuously or switched between the two states The latter technique makes use of the magnetic hysteresis of the ferrite core to store a remanent permeability The switching speed between the two remanent states is in the

order of few microseconds Because of the difficulty in magnetic field generation, the ferrite tunable devices are always bulky, slow and power consuming

1.2.2 Ferroelectric Technology

Ferroelectric based varactors have demonstrated strong potential for commercial applications in the microwave frequency range for its tuning speed, low cost and ease of power handling In comparison with non-ferroelectric technologies, ferroelectric varactors and tunable devices based on them have potential of easy integration with standard Si and GaAs processes [22-26] As opposed to ferrite, the permittivity of ferroelectric materials can be changed proportionally to the intensity of an applied DC electric field Permittivity also varies with temperature so it must be compensated within an acceptable range An operating temperature slightly above the Curie-point in the paraelectric phase is normally preferred for tunable devices as it is characterized by a less dispersive permittivity at microwave frequencies and low hysteresis effect Moreover, the permittivity and breakdown voltage of ferroelectric thin films are intrinsically high thus allowing for increased miniaturization and high power handling Overall, though ferroelectric materials may prevail or yield in different aspects of the contest, they have been proven to be a very competitive candidature for the development of microwave tunable devices

The main disadvantage in using ferroelectric materials for tunable wireless devices is the relatively high dielectric loss tangent of ferroelectric materials

which leads to microwave dissipation However recent researches, including work carried out in our lab [27-28], indicated that the loss tangent can be reduced by improved thin film fabrication method and material enhancement such as by doping [29-35] or multilayering the ferroelectric thin film [36-38] Furthermore with proper device design, it is usually possible to reduce insertion loss through reduction in device tunability such that a compromise can be made for satisfactory performance Although there are a lot of reports

on the integration of ferroelectric materials with tunable microwave devices, further improvement and understanding of ferroelectric materials is required before more competitive devices can be developed Also, research is required

to develop prototypes of ferroelectric based varactors and miniature microwave communication applications such as phase shifters and tunable matching networks, etc

1.3 Ferroelectric Materials and Their Microwave Applications

A ferroelectric material is normally in single crystalline, thin film or polycrystalline form, and possesses a reversible spontaneous polarization over

a certain temperature range There is a critical temperature (usually referred as the Curie or transition temperature), which marks the transition from an ordered to a disordered state The phase transition induces a mechanical strain, tending to change not only the volume and the shape of the material body, but also the refractive index of the material Thus ferroelectric materials exhibit piezoelectric, pyroelectric, and electro-optic properties in addition to the

ferroelectric property, which can be used for many technological applications [39]

In modern physics of dielectrics, we deal with the study of ferroelectric, anti-ferroelectric, piezoelectric and pyroelectric materials All ferroelectrics are piezoelectric and pyroelectric, but they additionally possess a reversible, non-volatile macroscopic spontaneous electric dipole moment in the absence

of an external electric field In simple words, ferroelectric crystals can be seen

as assembly of batteries with a particular orientation, which remains stable unless an external electric field is applied to change its direction Their polar state is a consequence of the structural transition from a high-temperature (high-symmetry) paraelectric phase to a low-temperature (low-symmetry) ferroelectric phase For the use in tunable microwave devices, the paraelectric phase is often preferred since it has no hysteresis associated with the domain walls

Two main types of ferroelectrics are distinguished: order-disorder and displacive In order-disorder type ferroelectrics, the ferroelectricity, i.e the spontaneous polarization is associated with the ordering of the ions below phase transition temperature Crystals with the hydrogen bounding, like

KH2PO4, belongs to this type of ferroelectrics In displacive ferroelectrics one sublattice of the crystal is displaced relative to the other resulting in spontaneous polarization below phase transition temperature Complex metal oxides with perovskite structure belong to this group (i.e BaTiO3, KNbO3) Most ferroelectric materials have perovskite structure, named after the CaTiO3

perovskite mineral; in fact ferroelectricity itself is closely related to the intrinsic structure frustration associated with perovskite structures A perfect

perovskite structure has a general formula of ABO3, where A represents a divalent or a trivalent cation, and B is typically a tetravalent or a trivalent cation As shown in Figure 1.1, A atoms occupy the corner of the cube, while the B atoms sit in the center inside the octahedral formed be oxygen atoms,

which are at the face centers Above the Curie temperature (T c), their crystal lattice has a cubic structure (Figure 1.1 (a)) In this phase the crystal has no spontaneous polarization Its permittivity is rather high, DC field, temperature

and strain dependent Below T c the centers of the positive and negative charges shift (Figure 1.1 (b)), and the crystal is characterized by spontaneous polarization

(a) (b) Figure 1.1 ABO3 perovskite lattice structures in paraelectric (a) and

ferroelectric states (b) Above T c, their crystal lattice has a cubic structure in (a)

and is paraelectric Below T c, the centers of the positive and negative charges shift and the crystal is characterized by spontaneous polarization

1.3.1 Theory of Dielectric Response of Ferroelectric Materials

The most straightforward description of the dielectric response of ferroelectrics is given by thermodynamic theory of Landau [40] The thermodynamic theory correlates different macroscopic values such as

temperature, polarization and energy The theory is based on the expansion of

the Helmholtz free energy F of a ferroelectric crystal as a function of the vector macroscopic polarization P [41] We assume that the free energy F in

one dimension may be expanded as:

𝐹(𝑃, 𝑇) = 12𝛼𝑃2+14𝛽𝑃4+ ⋯ (1.1) where the coefficients α, β depend, in general, on temperature At this instance the higher order terms in this expansion are ignored The series does not

contain terms in odd powers of P because the free energy of the crystal will

not change with polarization reversal The phenomenological formulation should be applied for the whole temperature range over which the material is

in the paraelectric and ferroelectric states

The equation of state 𝜕𝐹/𝜕𝑃 = 𝐸 then leads to a relation between the polarization and electric field:

𝐸 = 𝛼𝑃 + 𝛽𝑃3 (1.2)

From equation (1.2) it becomes clear that the coefficient α should have a meaning of the inverse permittivity:

𝛼 = 1/(𝜀𝜀0) , (1.3) where ε is the relative dielectric permittivity and ε0 is the dielectric constant of vacuum According to the Landau theory, the coefficient α is assumed to be

linear function of temperature and vanishes at the Curie-Weiss temperature T 0

To obtain the ferroelectric state, the coefficient of P 2 term must be negative for the polarized state to be stable, while in the paraelectric state it must be

positive passing through zero at some temperature T 0 :

𝛼 = (𝑇 − 𝑇0)/𝜀0𝐶 , (1.4)

where C is taken as a positive constant called the Curie-Weiss constant and the value of T 0 may be equal to or lower than the actual transition temperature T c

(Curie temperature) The validity of this assumption is experimentally supported by the Curie-Weiss law

If β is positive, the polarization for zero electric field can be found from eq

(1.2):

𝛼𝑃𝑠 + 𝛽𝑃𝑠3=0, (1.5)

so that either 𝑃𝑠 = 0 or 𝑃𝑠2 = (𝜀0𝐶/𝛽)(𝑇0− 𝑇) For 𝑇 ≥ 𝑇0, the only real root

of eq (1.5) is at 𝑃𝑠 = 0 since C and β are positive Therefore, T 0 is equal to the

Curie temperature T c For 𝑇 < 𝑇0, the minimum of the free energy in zero

electric field is at

𝑃𝑠 = �(𝑇0 − 𝑇)/(𝛽𝜀0𝜀𝐶) (1.6)

Below the Curie temperature T 0 the ferroelectric is in ferroelectric state with the spontaneous polarization 𝑃𝑠 = �(𝑇0− 𝑇)/(𝛽𝜀0𝜀𝐶) Above T 0 the ferroelectric is in paraelectric state with 𝑃𝑠 = 0 The transition is the first order

if β is negative For the first order phase transition, the Curie-Weiss

temperature is a little bit smaller than the Curie temperature, 𝑇0 < 𝑇𝑐

From eq (1.2) and eq (1.3), the dielectric permittivity is obtained:

In ferroelectric state (𝑇 < 𝑇𝑐) with zero bias electric field (𝑃 = 𝑃𝑠), the

dielectric permittivity is written as:

𝜀 =2(𝑇𝐶

𝑐 −𝑇) (1.9)

In ferroelectric state, the electric field response of ferroelectric materials shows a typical hysteresis loop as shown in Figure 1.2 (a); similar behavior is also found in ferromagnetic materials under magnetic fields The remnant

polarization P r, is the polarization value of the material at zero bias, also

known as the spontaneous polarization, the saturation polarization P s, is the

maximum polarization, and the coercive field E c, is the field necessary to reverse the direction of the net polarization of the material In paraelectric state, the spontaneous polarization is zero and the polarization dependence on the external electric field is again nonlinear but without hysteresis loop, as shown in Figure 1.2 (b)

(a) (b) Figure 1.2 Polarization dependences on the applied electric field for (a) a

ferroelectric state and (b) a paraelectric state In (a), P s is the saturation

polarization, P r is the remnant polarization, E c is the coercive field

The main attraction of ferroelectric materials is the strong nonlinear change of their dielectric permittivity ε on the application of an external

electric field This characteristic is commonly described by the tunability n

defined as the ratio of the dielectric permittivity of the material at zero electric field to its permittivity at some non-zero electric field, expressed as:

𝑛 = 𝜀(0)/𝜀(𝐸) (1.10) Another way of expressing the tunability is the relative tunability:

𝑛𝑟 = (𝜀(0) − 𝜀(𝐸))/𝜀(0) (1.11) The dielectric loss is a critical parameter to consider when optimizing the properties of the ferroelectric material, which should be taken into account in the device design Low loss is almost always desired for electronic applications, especially for high frequency devices, where dielectric losses would lead to signal loss, which decreases the signal to noise ratio Dielectric losses arise in ferroelectric crystals due to three predominant sources: (1) an intrinsic loss attributed to multi-phonon scattering, (2) a loss associated with the conversion of the microwave field into acoustic oscillations by regions with residual ferroelectric polarization, and (3) extrinsic losses due to motion

of charged defects such as interstitials and vacancies resulting in acoustic waves at the frequency of the applied field [5] The trend “the higher the dielectric constant, the higher the tunability, loss, and temperature dependence

of the dielectric permittivity” is observed for many dielectrics [42] The optimal balance of these parameters is one challenging problem for best device performance

1.3.2 Tunable Ferroelectric Thin Film Microwave Devices

Ferroelectrics, especially complex oxides with perovskite structure, are truly multifunctional materials The sensitivity of the physical properties

(permittivity, polarization, refractive index, magnetic permeability etc.) of these materials to temperature, external electrical, magnetic, and mechanical fields (stresses), especially near the temperatures of phase transitions, make them attractive for applications in electronic and optical devices [43] Previous designs for tunable microwave devices in bulk ferroelectric materials have resulted in low capacitances and very high applied voltages Thin-film ferroelectrics provide an advantage over the bulk materials for practical device applications, such as ferroelectric thin films provide us with possibility of device miniaturization and integration Additional advantage of thin films for tunable devices is the relative low applied electric field, since the voltage can

be applied in thickness direction which is usually not larger than 1 μm and small gap electrodes can be readily fabricated with the modern lithography techniques As a result, Ferroelectric thin films, such as barium strontium titanate (BST), have been extensively studied for microwave device applications This study will mainly focus on the applications of BST thin films on tunable microwave devices

Examples of the applications in the field of microwave engineering include varactors, tunable microwave resonators, phase shifters, tunable filters, voltage controlled oscillators, tunable diplexers, and tunable matching networks etc Many new communications systems would greatly benefit from these components For example, microwave phase shifters, one of the first and simplest components to be made with ferroelectrics, are used in antenna arrays

in order to produce a beam scanning function It is possible to integrate ferroelectric materials to produce complex electronically steerable antenna arrays with applications in planned low Earth orbiting satellite networks

1.4 Objectives of This Study

Ferroelectric thin film varactors have performances comparable or better than competing commercial technologies, such as semiconductor, MEMS Nowadays, there are an extensive number of reports on the integration of ferroelectric thin films for microwave applications Also, the new “old” technology, ferroelectric microwave devices, is making its way from-the-labs-to-the-fabs [44-45] However, there are still many challenges to be solved For example, the major disadvantage of using ferroelectric thin films for tunable microwave devices is its relatively high dielectric loss tangent which leads to microwave dissipation The microwave dielectric properties of ferroelectric thin films in the varactor form have not been accurately evaluated at microwave frequencies, and the electrical means of improving the total quality factor (Q factor) have not been implemented Furthermore, the developments

of various microwave systems, such as mobile wireless system and radar, have been the driving force behind substantial research efforts toward the designs of miniature and tunable microwave circuits using ferroelectric thin film

A high performance ferroelectric varactor would have exploitation potential Tunable microwave devices (such as phase shifters, filters, matching networks, etc.) based on ferroelectric varactors are the most representative components considered for applications in microwave systems Miniaturization of these devices is to reduce space and weight requirement and is desirable in applications where portability or high device density is required

Therefore, the main aim of this research was to develop prototypes of ferroelectric based varactors and miniature microwave communication applications such as phase shifters and tunable matching networks, including those based on the group of newly emerging artificial materials termed as metamaterials The specific objectives of this work are:

(a) To reduce the relatively high dielectric loss tangent of ferroelectric thin films through materials development; to develop accurate characteristic techniques for microwave properties of ferroelectric thin films

(b) To overcome the initial technological obstacle of fabricating parallel-plate varactors and its applications in tunable devices; to integrate the parallel-plate varactors into tunable microwave devices such as tunable matching network, tunable filter and phase shifter etc

(c) To understand and implement a new concept regarding metamaterials used

in phase shifters through transmission lines loaded with varactors and inductors; to demonstrate its advantages over the novel phase shifter and explore its potential in possible commercialization

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Chapter 2

FABRICATION AND MICROWAVE

CHARACTERIZATION OF FERROELECTRIC

THIN FILMS

2.1 Fabrication of Ferroelectric Thin Films

Most device fabrication requires sophisticated techniques for synthesizing high-quality oxide thin films to understand their unique physical properties and device performance Current methods of fabricating ferroelectric thin films include: RF sputtering [1-2], Sol-gel method [3-4], molecular beam epitaxy (MBE) [5], metal-organic chemical vapor deposition (MOCVD) [6], and pulsed laser deposition (PLD) [7-9] The first two provide polycrystalline

or amorphous thin films, while the others can provide highly oriented thin films Compared with other thin film deposition techniques, PLD shows several advantages, such as (1) the ability to reproduce the stoichiometry and crystallographic status of very complex bulk materials; (2) the relatively high growth rate of 1-5 Å/pulse and even higher; (3) an energy source independent

of the deposition environment; (4) no ultrahigh vacuum requirements; (5) wide range of ambient reactive gas pressure, typically from 10-9 to 1 mbar; (6) relative simplicity of the growth facility offering great experimental versatility, e.g using multi-targets and multi-component complex materials to produce multilayer or using dual-beam lasers to perform in-situ doping; and (7) reduction of film contamination due to the use of light for promoting ablation

Today, PLD technique has become one of the most successful vapor deposition techniques in research and device applications Besides all the advantages, there exist two main disadvantages for PLD technique: (1) splashing effect that causes particulate and droplets on the film surface; (2) narrow angular distribution of the species in the plume, which makes it difficult to fabricate large area film Despite of the above drawbacks, PLD is

an effective research tool because of its versatile deposition capability and ease of stoichiometry control In this study, PLD is utilized to grow oxide thin films including BST thin films and electrode layer La0.7Sr0.3MnO3 etc Metal electrodes are deposited by RF magnetron sputtering and electroplated

Figure 2.1 The schematic basic setup for PLD system for thin film fabrication

2.1.1 Pulsed Laser Deposition Process

Pulsed laser deposition (PLD) is a physical vapor deposition technique where a high-power pulsed-laser beam is focused inside a vacuum chamber by

a lens to strike a target of the desired composition Figure 2.1 is a schematic diagram of the PLD system used in this project, where the whole setup includes a laser beam source and a stainless steel vacuum chamber with a rotating target holder and a substrate stage with a programmed-temperature controller While the basic setup is simple compared to many other deposition techniques, such as vapor chemical deposition, magnetron sputtering, etc, the physical phenomena of laser-target interaction and film growth are quite complex Details regarding its principles can be found from a number of sources in the literature [10] For the laser system, the commercial LAMBDA PHYSIK excimer laser system is normally used to produce the KrF excimer high-energy laser The laser beams are then guided using coated total reflection mirrors After passing through a UV SiO2 window on the chamber wall, the beams are finally focused onto the rotating ceramic target inside the high vacuum chamber

The target holder is customized such that it can hold up to 2 different targets inside the chamber This would enable us to grow different thin film layers without breaking the vacuum by rotating the holder to the desired target [11] This would also help to keep the sample clean during the deposition The rotation of the target during deposition to minimize the large particulate splashing effect and achieve a more uniform ablation of the target is driven using a small DC motor Opposite the target holder, the substrates can be mounted on a stainless steel substrate heater in which the resistive filaments

are embedded Silver paste is used to hold the substrates onto the holder and also for the efficient heat transfer between the substrates and the heater A turbomolecular pump in combination with a mechanical pump is used to pump down the deposition chamber to a background pressure of less than 10-6 mbar The reactive gas, O2 in the case of fabricating oxide thin film, is injected into the chamber through a nozzle placed close to the target Prior to the deposition

of thin films, the target surface was polished using sand paper and sputtered in PLD chamber The substrates were first washed in the ultra-sonic bathing in the acetone for 10~15 minutes with the aim to remove oil molecular, fibers, and other contaminants on the surface In order to have a clear picture

pre-of the PLD technique, one can divide the processes pre-of PLD into three main stages: (1) interaction between laser and ceramic target surface, (2) interaction between species in the plasma plume and ambient gas, and (3) film growth on the substrate surface As the process is repeated with more laser pulses, a thin film forms on the substrate surface, as shown in Figure 2.2

Figure 2.2 The shape of the plasma plume in PLD process

2.1.2 Target Preparation and Thin Film Deposition Parameter

Optimization

Most of the targets used in PLD were prepared using solid-state reaction method Solid-state reaction is the most conventional synthesis method for preparing the multi-component targets In this thesis, Ba0.5Sr0.5TiO3 and

Ba0.25Sr0.75TiO3 targets with the diameter of about 2.5 cm were prepared using BaTiO3, SrTiO3 powders with ratio 1:1 and 1:3, respectively The BaTiO3 and SrTiO3 powders were mixed and calcined at 950 °C for 1 h before they were compacted and sintered at 1350 °C for 4 h As the BST thin films fabricated in this thesis were for the fabrication of microwave devices, the deposition parameters were optimized for the best crystal quality Ba0.5Sr0.5TiO3 had the maximum tunability, as well as relatively high dielectric loss at room temperature Ba0.25Sr0.75TiO3 exhibited low dielectric loss and permittivity well suited for the fabrication of low capacitance elements in devices For comparison, the BST thin films with the different Ba/Sr ratio were grown on platinized silicon (Pt/TiO2/SiO2/Si) substrates and (100) LaAlO3 (LAO) single crystal substrates using PLD with ceramic targets of Ba0.5Sr0.5TiO3 and

Ba0.25Sr0.75TiO3, respectively The BST thin films were deposited with the following optimized parameters: the distance between target and substrate is about 4.5 cm; a KrF excimer laser (λ=248 nm) of 3 Hz with the energy density

of 1.5 Jm-2 was used; the temperature of substrate and oxygen pressure during the growth of BST films were kept at 700 °C and 0.2 mbar, respectively; the deposition time is about 30 minutes; after deposition, the samples were annealed for half an hour at about 650 ˚C in 1×103

mbar oxygen pressure